The present invention relates to windscreen wipers in general. In particular, it concerns wipers which move linearly across a surface to be wiped and wipers for preventing drag-back of water during wiping of a surface.
It is known that a wiper travelling linearly across a generally rectangular surface can wipe a much larger proportion of the surface than a wiper with the conventional arcuate motion. However, wipers of this type are not currently in extensive use. This is mainly due to structural problems with the mechanism for producing the required linear reciprocating movement.
U.S. Pat. No. 1,665,570 to Waters, U.S. Pat. No. 1,748,055 to Brown et al. and more recently U.S. Pat. No. 5,068,942 to Vrettos describe examples of the predominant mechanism used in conventional linear wipers, hereinbelow referred to as the endless-belt mechanism. The endless-belt mechanism includes an endless belt or chain positioned along one side of a window, and a wiper attached by a rotatable bracket to a point on the endless belt. When the endless belt is driven continuously in one direction by an electric motor, the wiper is pulled to-and-fro across the surface. The endless-belt mechanism has numerous structural problems. There is a tendency for the belt to stretch, resulting in failure of the drive mechanism. Belts made from rubber, synthetic polymers or fabric generally have a very limited lifetime due to wear over the rollers at each end. Use of a metal chain generally causes unacceptable noise. And when repairs are needed, the entire belt will generally need replacing and refitting.
An alternative mechanism is described in U.S. Pat. No. 1,561,320 to Glidric et al. in which an electric motor mounted on a wiper is used to drive the wiper along a stationary track. A switch is used to reverse the polarity of the power supply when the wiper reaches each end of the window. This design has the advantage that it uses a static track which requires much less maintenance than a moving belt. However, this mechanism puts great strain on the motor by abruptly reversing the polarity of the power supply while the motor is rotating rapidly and has the momentum of the entire wiper assembly tending to maintain its rotation. Since this happens every time the wiper reaches the end of its motion, the motor is likely to deteriorate rapidly requiting frequent maintenance. There is also a delay at each end of the movement due to the time taken by the motor to accelerate to its full speed.
There is therefore a need for an apparatus which has a reliable mechanism for driving a wiper linearly across a window.